1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal display device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for minimizing deterioration of an image by gray inversion.
2. Discussion of the Related Art
As the information society develops, demand for flat panel display devices is gradually increasing. To satisfy this demand, various types of flat panel display devices, such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electro-luminescent display (ELD), a vacuum fluorescent display (VFD), and the like are being researched and produced for use as a display device in a variety of apparatuses. The LCD is the portable display device most widely used as a substitute for cathode ray tube (CRT) due to the LCD having the features of high image quality, lightweight, thin profile, and low power consumption. The LCD is been used in various fields, such as TVs, desktop computer monitors and laptop computer monitors. Lightweight, thin profile, and low power consumption as well as high definition, high brightness, large screen size and a high quality image are features that will enable the LCD to be used as a general display device in numerous applications.
The LCD displays an image by controlling electric field across liquid crystal molecules in liquid crystal cells to modulate light transmission through the liquid crystal cell. LCDs can either be a vertical electric field type liquid crystal display device or a horizontal electric field type liquid crystal display device depending on the direction of applied electric field for driving the liquid crystal. In the vertical electric field type liquid crystal display device, a common electrode on the upper substrate and a pixel electrode on the lower substrate face each other in vertical direction such that electric field is created in a vertical direction across the liquid crystal by an electric voltage applied to the electrodes. The vertical electric field type liquid crystal display device has a wide aperture ratio but has a narrow viewing angle. A typical liquid crystal mode of the vertical electric field applied type liquid crystal display device is a twisted nematic (hereinafter referred to “TN”) mode.
FIGS. 1A and 1B are cross-sectional views respectively illustrating orientation of liquid crystal molecules in an OFF-state and an ON-state of a TN mode LCD. In the liquid crystal panel 10 of the TN mode LCD, as shown in FIGS. 1A and 1B, a liquid crystal layer 5 of liquid crystal molecules is positioned between an upper substrate 7 and a lower substrate 3. For illustrative convenience, FIGS. 1A and 1B illustrate liquid crystal molecules seen from a three O'clock direction. An upper polarizer 9 having a light transmission axis of a specific direction is attached on a light emission surface of the upper substrate 7 and a lower polarizer 11 having a light transmission axis perpendicular to the light transmission axis of the upper polarizer 9 is attached on a light incident surface of the lower substrate 3. Under the assumption of a normally white mode in which WHITE is in ‘OFF’-state and BLACK is in ‘ON’-state, operation of a TN mode will be described in reference to FIGS. 1A and 1B.
In the OFF-state, as shown in FIG. 1A, when electric voltage is not applied to the upper substrate 7 and the lower substrate 3, local optical axes (directors) are continuously twisted between the upper substrate 7 and the lower substrate 3 by 90 degrees. In this OFF-state, the polarization characteristics of linear polarized incident light transmitted through the lower polarizer 11 is changed and is transmitted through the upper polarizer 9.
In the ON-state, as shown in FIG. 11B, when electric voltage is applied to a common electrode on an upper transparent substrate and a pixel electrode on a lower transparent substrate such that an electric field is applied across the liquid crystal molecules due to the voltage difference between the common electrode and the pixel electrode, an axis of an intermediate portion of the liquid crystal layer 5 is no longer aligned to transmit light. In this ON-state, the linearly polarized incident light transmitted through the lower polarizer 11 is transmitted through the liquid crystal layer 5 and the polarization characteristics thereof are maintained so that the linearly polarized incident light is not transmitted through the upper polarizer 9.
FIG. 2 is experimental data illustrating changes of brightness according to the angle viewed of the viewing angle, and FIGS. 3A and 3B are photographs respectively illustrating a gray inversion image and a normal image. The above-described TN mode has a high permeability and is easy to produce, but gray inversion occurs in lower angles of the viewing angle. The gray inversion, as shown in the experimental data in FIG. 2, illustrates a phenomenon in which a low gray level is brighter than a high gray level in the lower angles of the viewing angle. As seen in FIG. 3A, the image quality at the lower angles of the viewing angle have a gray inversion that significantly deteriorates the image as compared to a direct view, as shown in FIG. 3B. The reasons for the gray inversion occurring in the TN mode will be explained in reference to FIGS. 4A and 4B.
FIGS. 4A and 4B are views respectively illustrating changes in light transmission of the liquid crystal in the OFF-state and the ON-state of a liquid crystal according to the angles viewed of the viewing angle. A big reason for the gray inversion is because the reflection index is changed according to the angle of viewing. Birefringence dΔn of the liquid crystal layer is thickness d of the liquid crystal layer times the reflective index Δn. As shown in FIG. 4A, in the OFF-state of the TN mode liquid crystal panel 10, there is almost no difference between a first birefringence dΔn1 in a first direction (the lower angle of viewing) that a user sees an image from the lower side of the liquid crystal panel 10, a second birefringence dΔn2 in a second direction (the upper angle of viewing) that the user sees the image from the upper side of the liquid crystal panel 10, and a third birefringence dΔn3 in a third direction that the user sees the image from directly in front of the liquid crystal panel 10.
In contrast, as shown in FIG. 4B, since an average director A of the liquid crystal is slightly inclined from the vertical direction in the ON-state of the TN mode, the effective birefringence dΔn of a light transmitted through the liquid crystal is slightly different according to an angle of viewing. More precisely, the relationship of dΔn1<dΔn3<dΔn2 is established. In other words, since the user sees a short axis and a long axis in the first direction of the liquid crystal when changing to the third direction and to the second direction, the gray level to be actually implemented is approximately implemented in the first direction but the gray level is at a slight higher brightness in the second direction and in the third direction as the value of dΔn increases (P1 area in FIG. 2). Thus, when the user looks at the image in a fourth direction having an angle of viewing lower than that of the first direction, the gray level in the fourth direction gray is brighter than the gray level as seen in the first direction (P2 area in FIG. 2) since dΔn4 in the fourth direction is greater than dΔn1 in the first direction.
FIG. 5 illustrates gray inversion regions where the gray inversion is generated according to the angles of viewing. As shown in FIG. 5, on the basis of a point where the average direction Δn of the liquid crystal is theoretically “0” (zero), the birefringence of the low gray level is greater than the birefringence of the high gray level at the lower angle of viewing from the lower part of the panel (at the lower side of the liquid crystal panel), and thus a gray inversion appears in the image. This gray inversion deteriorates the image quality in the TN mode and restricts the use of the display by the user.